On Jan. 30, 1962, three girls began to laugh in the middle of a lesson at the Kashasha School. The teacher sent the laughing girls out into the yard to restore order in her classroom. This was a Christian boarding school, and the teachers expected order. They insisted on it.

The laughter began without a spark—nothing anyone else could see or hear. Out in the yard, the three girls continued to erupt in a helpless burst of giggling. Other students watched the girls act out their madness, whatever this was.

One by one, the onlookers began to join in the laughter. But there was no humor in their eyes. This was laughter like a curse—as a local Muslim cleric told an American journalist 50 years later, it was as if the spirits of the ancestors were showing their power.

The laughter spread. Almost 160 students between the ages of 12 and 18 attended the school. By the end, 95 of them had succumbed to the epidemic, which was anything but joyful. Along with the giggles, the girls wept uncontrollably. They fought hard when adults tried to restrain them. They said things were moving in their minds, ungovernable things. Officials closed the school and sent the afflicted girls home to their villages.

The laughter spread further. From Kashasha to the village of Nshamba, through a school near Bukoba, even over the borders of the nation, which was called Tanganyika then. It bled into neighboring Uganda. It leaped from village to village like a forest fire.

The only contemporary account of the phenomenon is a 1963 report by P.H. Philip, a local medical officer, and A.M. Rankin, a professor at Makerere University College. According to their article, “An Epidemic of Laughing in the Bukoba District of Tanganyika,” the laughing sickness went on for six months. Other sources say it went on for a year, or two, or even more. About 1,000 people contracted the “illness,” many of them young women and girls.

The local Bahaya people called it Endwara Yokusheka, or “the illness of laughing.” Later Tanzanians would call it omuneepo, “the laughing disease.”

No one knows why it started, how it spread, or what invisible switch finally silenced the afflicted. But people have theories.

The Theory of Contagion

When he first started studying laughter, psychology professor Robert Provine went into the field with a handheld tape recorder. He and his collaborators recorded more than 1,000 real-life “laughter episodes” and studied the circumstances surrounding them.

For the most part, Provine discovered, people weren’t laughing because something was funny. They used laughter as a sort of social glue, a message of peace and group cohesion. Laughter was social, Provine concluded. And it was contagious.

“The Tanganyikan laughter epidemic is a dramatic example of the infectious power of laughter,” he wrote in an article for American Science. “[Which is] something that many of us may have experienced in our own lives.”

While it’s true that we’ve all caught the giggles from a friend, few of us have experienced contagious laughs on the scale of the Tanganyika episode. Could the explanation be as simple as Provine suggests?

A Virus in the Brain

Silvia Cardoso is a behavioral biologist at State University of Campinas in Brazil. She studies laughter, both in humans and animals (yes, animals laugh; search it). Cardoso wrote about the Tanganyika Laughter Epidemic from a unique perspective. Unlike most of her colleagues, who favor the sociogenic illness theory, Cardoso believes Endwara Yokusheka can be traced back to viral infection.

She shared her theory in a 2002 interview with New Scientist. “I find it improbable that a purely psychological mass reaction would last so long and be so widespread,” she said.

Sometimes people have fits of abnormal, inappropriate, unrestrained, and uncontrollable laughter dissociated from any social or humor stimulus. American neurologists Hanna Damasio and Antonio Damasio suggest that abnormal laughter occurs when structures in the basal part of the brain are damaged. Based on this model, they suggest that a viral infection, probably some kind of encephalitis in the basal part of the brain, provoked the 1962 epidemic.

This position contradicts Rankin and Philip, who failed to find any evidence of viral infection or food toxicity among the sufferers.

The Psychological View of the Tanganyika Laughter Epidemic

The truth behind the 1962 episode remains lost in the shadows of history. No one wrote down the names of the three girls at the center of the outbreak. Accounts vary as to how long it lasted, how many people were involved, and the ultimate cause of the laughter.

Scientist Peter McGraw and journalist Joel Warner flew to Tanzania in search of first-hand accounts of the Tanganyika Laughter Epidemic. They detailed the quest in a 2014 book, The Humor Code, to which this article is deeply indebted.

Warner and McGraw visited the schoolhouse where the epidemic began. They talked to locals about their memories of the event. They even found one woman, now in her seventies or eighties, who might have been one of the victims. She refused to speak about omuneepo.

In the end, McGraw and Warner had to draw their conclusions from the literature, just like anyone else. They point to researcher Christian Hempelmann’s 2007 investigation into Endwara Yokusheka, published in Humor: International Journal of Humor Research. Hempelmann concluded that “the event is a motor-variant case of mass psychogenic illness of which laughter is one common symptom.”

Mass psychogenic illness (which psychologists used to call “mass hysteria”) is essentially a stress response, common to groups of people who feel powerless. Researcher John Waller explains the condition in a comprehensive overview published in the journal Psychologist.

The students of Kashasha School probably did experience severe “psychological tension.” They were away from home for the first time. After a childhood of permissive cultural norms, they were thrust into the strict world of the Christian mission. Tellingly, “mass motor hysteria usually occurs in schools,” Waller wrote. He goes on to cite the Tanganyika Laughter Epidemic as an example, noting that:

“Schools in central Africa are especially prone to outbreaks of mass motor hysteria. Late in 2008 several girls in a Tanzanian school responded to the pressure of taking important exams by dissociating: some fainted, while others sobbed, yelled or ran around the school.”

The first scientists to document the Tanganyika Laughter Epidemic came to a similar conclusion. “It is suggested that this is mass hysteria in a susceptible population,” wrote Rankin and Philip in 1963. “This is probably a culturally determined disease.”

The internet is intrigued by the Tanganyika Laughter Epidemic. It seems even joyful in a way, but in fact, the story is about the destructive force of hopelessness and the body’s final revolt against the boot heel of an indomitable power structure, leaving it as an eerie, indescribable event in history.